This invention pertains to a method of preparing an oxidation catalyst containing gold and titanium.
Catalysts containing gold and titanium are useful in the hydro-oxidation of olefins to olefin oxides. For example, it is known to oxidize propylene with oxygen in the presence of hydrogen and a catalyst containing gold and titanium to produce propylene oxide. Propylene oxide is a commercially important raw material for the production of propylene glycols and polyether polyols which are used in preparing polyurethanes.
The catalyst used in the aforementioned hydro-oxidation process more specifically comprises gold on a titanium-containing support. The support may be selected, for example, from titanosilicates, titanium dioxide, titanium dispersed on silica, and certain metal titanates. Optionally, the catalyst can further contain a promoter metal, such as an alkali, alkaline earth, or lanthanide rare earth metal, for the purpose of enhancing catalytic performance. Prior art representative of this process and catalyst composition includes PCT patent publications WO 98/00413, WO 98/00414, and WO 98/00415.
It is known to prepare a hydro-oxidation catalyst comprising a platinum group metal on a titanosilicate support by impregnation methods. Art of this type, represented by PCT patent publication WO 96/02323, discloses impregnating a solution of a platinum group metal salt onto a titanosilicate support, and thereafter reducing the impregnated support under hydrogen to adjust the bond energy states of the platinum group metal. Reduction under hydrogen disadvantageously may require high temperatures, and the reduction cannot be controlled sufficiently well.
Other art, such as EP-A1-0,709,360, teaches catalysts comprising ultra-fine particles of gold deposited on titanium dioxide being prepared by a deposition precipitation method. This method involves preparing an aqueous solution of a soluble gold salt, adjusting the pH to between 7 and 11, and then adding titanium dioxide to the solution. The resulting composite is calcined to obtain ultra-fine elemental gold particles deposited on the titanium dioxide carrier.
An alternative, but related method, exemplified in U.S. Pat. No. 4,839,327 and EP-A1-0709,360, involves a coprecipitation method. Here, an aqueous gold solution with a pH value of between 7 and 11 is added dropwise to an aqueous solution of a soluble titanium salt adjusted to the same pH range, so as to form a coprecipitate. The coprecipitate is calcined to obtain metallic gold deposited on titanium dioxide.
In another deposition precipitation method, exemplified by U.S. Pat. No. 4,937,219, a catalyst comprising ultra-fine gold particles immobilized on a mixed alkaline earth-titanium oxide is prepared. The preparation involves dissolving or suspending an alkaline earth-titanium compound, such as strontium titanate, in an aqueous solution of a gold compound, adjusting the pH to between 7 and 11, and adding dropwise a reducing agent thereby causing ultra-fine gold particles to be precipitated onto the alkaline earth titanate. The reducing agent is disclosed to be formalin, hydrazine, or citrate salts. A variation of this method is found in U.S. Pat. No. 5,051,394, wherein the pH of an aqueous solution containing a gold compound and a water-soluble titanium salt is adjusted with an alkali compound to yield a coprecipitate, to which is added a carboxylic acid or salt thereof. The coprecipitate thus treated is heated to form the catalyst comprising metallic gold deposited on titanium oxide.
All of the aforementioned deposition precipitation and coprecipitation methods suffer from multiple disadvantages. Specifically, the methods of the prior art require the accurate control of deposition conditions over a long period of time. Moreover, when a reductant is used, the gold particles may be reduced in solution before adhering to the support, which results in an inefficient use of gold. Since there is poor control over the precise amount of gold which is deposited onto the support, additional efforts are required to recover unused gold from the deposition solution. Even more disadvantageously, the prior art methods are temperature sensitive. They also require the use of large quantities of solvents and control of pH. Finally, the prior art methods may result in poor adherence of the gold particles onto the support.
In view of the above, it would be desirable to discover a simple, efficient, and reproducible method of preparing an active oxidation catalyst comprising gold deposited on a titanium-containing support. It would be desirable if the process avoided the disadvantages of the deposition precipitation and coprecipitation techniques. It would be more desirable if the process could be adapted to practical forms of the catalyst, such as, pelleted and extruded titanium-containing supports. It would be even more desirable if the process did not require a gold recovery stage. Such a process would advantageously reduce catalyst preparation efforts and costs, and equally importantly, conserve gold.
This invention is a process of preparing a catalyst composition comprising gold on a titanium-containing support. The process comprises impregnating a gold compound and a reducing agent onto a catalyst support under conditions sufficient to prepare the catalyst composition. Optionally, the impregnated support may be heated prior to use. Since the catalyst of this invention comprises titanium, a source of titanium must be present in the process of preparing the catalyst. This requirement is fulfilled when the support and/or the reducing agent contains titanium. Accordingly, the words xe2x80x9ca titanium-containing support,xe2x80x9d which are used to describe the catalyst, broadly embrace embodiments in which the support originally contained titanium, such as is found in titania, a titanosilicate, or a metal titanate; or alternatively, embodiments in which the titanium derived from the reducing agent has been dispersed onto a support which originally did not contain titanium, for example, titanium dispersed on silica; or alternatively, embodiments in which titanium derived from the reducing agent has been dispersed onto a support which originally contained titanium, such as, titanium dispersed on titania.
The invention described hereinabove beneficially provides a simple, efficient, and reproducible method of preparing an oxidation catalyst comprising gold deposited on a titanium-containing support. Advantageously, the method of the invention employs simple impregnation techniques, rather than the complex and time-consuming deposition precipitation and coprecipitation techniques of the prior art. Moreover, as compared with prior art methods, the method of this invention beneficially employs lesser amounts of solvents and does not require control of pH. Even more advantageously, the method of this invention provides better control over the amount of gold deposited onto the support. Since the method of this invention efficiently utilizes gold, it is not necessary to recover unused gold as is required in the prior art methods. As a further advantage, the method of the invention can be employed to prepare practical forms of the catalyst, namely, catalysts prepared with pelleted and extruded supports. Finally, in the method of this invention, reduction is simply accomplished by impregnating a reducing agent onto the support, as opposed to the prior art method of reducing under hydrogen. All of the aforementioned advantages provide a method of preparing the catalyst which is cost effective and more suitable for commercial purposes.
In another aspect, this invention is a catalyst composition comprising gold on a titanium-containing support. The catalyst is prepared by the aforementioned method comprising impregnating a gold compound and a reducing agent onto a catalyst support, wherein the reducing agent and/or the support comprise titanium, the impregnation being conducted under conditions sufficient to prepare the catalyst composition. Optionally, the catalyst may be heated prior to use.
As noted hereinbefore, the catalyst comprising gold on a titanium-containing support finds utility in the hydro-oxidation of olefins to olefin oxides. The term xe2x80x9chydro-oxidationxe2x80x9d means that the olefin oxidation is conducted with oxygen in the presence of hydrogen to form the olefin oxide. Water is produced as a coproduct of this process, but water can also be produced by the direct combustion of hydrogen. When the gold-titanium catalyst is prepared by preferred methods of this invention, the catalyst beneficially produces lesser amounts of water in the hydro-oxidation process, as compared with catalysts of similar composition prepared by the prior art methods. Illustrative of the aforementioned oxidation process is the hydro-oxidation of propylene to propylene oxide using a catalyst comprising gold on a titanium-containing support. When the catalyst is prepared by the method of this invention, the catalyst advantageously produces propylene oxide in a selectivity of greater than about 80 mole percent at a propylene conversion of at least about 0.2 mole percent.